Considerable evidence supports the idea that oversupply of dietary fat exceeds the storage capacity of adipose tissue and leads to ectopic lipid accumulation resulting in metabolic stress in skeletal muscle, liver, pancreas and possibly other tissues, leading to insulin resistance. One prevailing theory is that impaired skeletal muscle fatty acid oxidation (FAO) leads to the cytosolic accumulation of lipid intermediates that are directly linked to defects in insulin signaling. Others report lipid oversupply via a high fat iet can actually increase FAO to the extent that carnitine and TCA cycle intermediates are limiting, leading to mitochondrial abnormalities and skeletal muscle insulin resistance. Thus, evidence exists that both lipotoxicity and mitochondrial dysfunction contribute to skeletal muscle insulin resistance. Determining if and how these are intertwined is one of the hottest topics in type 2 diabetes research, with the fundamentally important question being: Does inhibition of FAO in skeletal muscle contribute to insulin resistance? To address this question we created mice lacking Carnitine Palmitoyltransferase-1b (CPT-1b) in muscle (CPT-1bm-/-). As predicted, CPT-1bm-/- mice have decreased mitochondrial FAO, increased IMCL, increased circulating free fatty acids (FFA) and triglycerides (TG), and decreased physical activity and exercise endurance. However, CPT-1bm-/- mice are not insulin resistant and have decreased circulating insulin and glucose, improved insulin and glucose tolerance, increased pyruvate oxidation, and increased whole body carbohydrate oxidation. At first glance, the lack of insulin resistance in spite of having hallmark predictors of the disease is at odds with prevailing lipotoxic theories. Indeed, it indicates that CPT-1bm-/- mice undergo unique adaptations to maintain insulin sensitivity in the face of decreased skeletal muscle FAO. Preliminary studies reveal potentially significant alterations promoting lipid uptake and storage, mitochondrial biogenesis, enhanced peroxisomal FAO, and stimulation of factors linked to the mTor signaling cascade. Specific Aim 1: Employ dietary and genetic manipulations in CPT-1bm-/- mice to gain a better understanding of acute and chronic consequences of mitochondrial FAO inhibition. Specific Aim 2: To evaluate the effects of decreased CPT-1b on glucose and fatty acid uptake and storage, mitochondrial number and function, and peroxisomal FAO. Specific Aim 3: To investigate how energy deficit signals are transduced through nutrient sensitive pathways to influence insulin sensitivity. These innovative studies will test the lipotoxic hypothesis and the mitochondrial overload hypothesis in a more definitive manner, providing critical mechanistic information on the role of CPT-1b and FAO in mitochondrial function and insulin resistance.